Trimethylolpropane, referred to hereinbelow as TMP, is a trihydric alcohol of great industrial significance which finds use, for example, in the field of coating resin, powder coating, foam and polyester production.
Customarily, TMP is prepared from n-butyraldehyde and formaldehyde. Base-catalyzed aldol reactions initially generate 2,2-dimethylolbutyraldehyde in the 1st reaction step from n-butyraldehyde and two equivalents of formaldehyde. 2,2-dimethylolbutyraldehyde may then be converted in the 2nd reaction step, for example, by reaction with further formaldehyde and base in a Cannizzaro reaction to TMP-containing reaction mixtures in which the formate of the base used also occurs as a further reaction product.
The reaction steps 1 and 2 may either be carried out separately or in one working step. The bases used both for the base-catalyzed reaction step 1 and also for the reaction step 2 which is stoichiometric in relation to the base quantity may optionally each independently be, for example, alkali metal or alkaline earth metal hydroxides or carbonates, or tertiary amines.
When, for example, a tertiary amine is used for the separate reaction step or the reaction steps 1 and 2 carried out in a single working step, this is known as the organic Cannizzaro process. When inorganic bases, for example alkali metal or alkaline earth metal hydroxides or carbonates, are used, this is accordingly known as an inorganic Cannizzaro process. The different physical and chemical properties of the ammonium formates or inorganic formates occurring as the further reaction products require very different work-up methods for the TMP-containing reaction mixtures.
The inorganic Cannizzaro process has the advantage that the TMP occurs in good purity and when, for example, calcium hydroxide is used as the base, the by-product is calcium formate. The resultant calcium formate may be used in various applications, for example, as an additive for animal nutrition, in the building materials industry, as an assistant in the leather industry, as an assistant in producing high-gloss papers, for treating scrubbing water and in smoke desulphurization.
The TMP-containing reaction mixtures obtainable by the Cannizzaro process generally have strong coloration caused by impurities. This coloration, which may be evaluated by a color number according to APHA (American Public Health Association) or Hazen, interferes in some uses. For this reason, the work-up customarily consists of cleaning operations, for example acid treatments, extractions and/or multistage distillations. Such multistage distillations generally require expensive, space-consuming and column arrangements which are costly and inconvenient in terms of apparatus and are accordingly quite unattractive in economic terms.
Various techniques are known for the work-up of TMP-containing reaction mixtures from inorganic Cannizzaro processes. For example, German/DE patent DD-P-45 078 describes a process for work-up in which TMP-containing reaction mixtures obtained from inorganic Cannizzaro processes are admixed with secondary cycloaliphatic alcohols which form azeotropes with water. Then water is azeotropically distilled off together with this alcohol, and the precipitated alkali metal or alkaline earth metal formates are removed by filtration. After distilling off the excess alcohol, the crude TMP obtained is distilled for further purification.
DD Patent 287 251 discloses a process for removing boiling components or “high boilers” (as commonly referred to) from TMP. Examples of high boilers include subsequent reaction products of TMP, in particular formals, which have a higher boiling point than TMP and accordingly accumulate in the distillation residue when crude TMP is vacuum-distilled. In the process described, the addition of from 0.02 to 0.05 kg of acid/kg at least partially converts many of the high boilers back to TMP. In accordance with DD Patent 287 251, this process has been found to yield an increase in purified TMP.
GB 1 290 036 also describes a process for decomposing high boilers in TMP-containing reaction mixtures which have been obtained by the inorganic Cannizzaro process. This involves adding cationic exchange resins and heating to from 50° C. to 140° C. to convert any formals present, which have a similar boiling point to TMP and tend to decompose at the boiling point of TMP, to products having other boiling points which can be easily distilled off.
U.S. Pat. No. 3,097,245 to Russell et al. describes a process for preparing TMP having an APHA color number of from 50 to 200. This color number is achieved by limiting the reaction time to less than 7 hours, acidifying the reaction mixture to a pH of less than 7 and limiting the concentrations of the starting compounds to from 5 to 20% by weight. The reaction is also followed by treatment of the solution obtained with cationic exchange resins and strongly basic quaternary ammonium anion exchangers.
Common to all the processes mentioned is that chemical treatment methods have to be carried out which worsen both the eco-balance and the economic preparability of the product or entail considerable apparatus, and accordingly financial, costs and inconvenience to generate products having an acceptable color number.
While the art is replete with various processes to purify TMP, there remains a need to provide an efficient process which makes it possible to obtain pure TMP in high yield and low color from the TMP-containing reaction mixtures prepared by the inorganic Cannizzaro process. The term “low color” herein refers to a very low APHA color number, i.e., of 50 or less, typically a Phthalic (phthalic anhydride) color less than 0.4 or 0.20, and/or an ACID WASH color less than 8 as measured by the ASTM standard procedures listed below and described hereinafter.                a) ASTM, Annual Book of ASTM Standards (1987), Section 6, Volume 6.03 (Paint) Standard D 1544-80, pages 284-286.        b) ASTM, Annual Book of ASTM Standards (1961), Section 8, Standard D 1544-58T, pages 728-30.        c) “Calibration of HunterLab Color QUEST for Gardner Color”, SJM 07-97.        
This invention thus relates to a process for obtaining low reacted color (interchangeably referred to herein as “low color”) trimethylolpropane (TMP). As previously mentioned, trimethylolpropane is produced by the condensation and cross Cannizzaro reactions of n-butyraldehyde and formaldehyde in the presence of a strong alkaline hydroxide, such as caustic solution. Removal of the TMP from the reactor solution can be carried out by the partition of the product between the aqueous solution and an organic solvent for the TMP, e.g., ethyl acetate, isobutanol, butyl acetate, and the like. The TMP is then removed from the organic layer, as for example by distillation. Another method for the removal of the TMP from the organic layer and the further removal of caustic from the TMP is to add a second solvent, one in which the TMP is insoluble, in an amount sufficient to separate the aqueous but not sufficient to separate out the TMP, remove the water layer, and then remove the first solvent by distillation. Typically the second solvent is a non-polar solvent, e.g., xylene. The TMP/second organic solvent mixture is allowed to settle and the separated TMP is removed by decantation. This latter process is set forth in Palmer et al., U.S. Pat. No. 3,956,406. These processes generally produce a product which has an acid-wash color of about 5-10 Gardner Units (GU) or phthalic anhydride color of about 100-300 APHA. Historically, the art has obtained low reacted color TMP by extracting, leaching, or further purifying color-causing impurities generated in the reaction to produce TMP.
U.S. Pat. No. 5,603,835 to Cheung et al. discloses a process which extracts color-causing impurities generated in the reaction to produce TMP. The process comprises extracting purified final TMP with an organic solvent wherein the color bodies are soluble in, but not the TMP. This extraction is reported to result in greater than about 85% yield of TMP product having an acid-wash color of 3 or less Gardner Units. The phthalic anhydride color of the TMP product is also reported to be less than about 100 APHA. It is important to note that low color TMP is not analogous to high purity TMP. The acid wash color of TMP is determined by extracting TMP with an organic solvent, followed by washing the extract with sulfuric acid, and monitoring the reacted color, e.g., through a colorimeter, to obtain a Gardner Unit (GU) value. The purity of a TMP sample is generally increased/enhanced by (re)crystallization techniques, or the like. Recrystallization may remove salts generated by the reaction of formaldehyde and butyraldehyde, but not necessarily remove color body impurities found in the TMP product. Palmer et al. (U.S. Pat. No. '406) addressed enhanced sample purity of TMP, while Cheung et al. (U.S. Pat. No. '835) addressed enhanced color of TMP. Both U.S. Pat. No. '406 and U.S. Pat. No. '835 involve a type of extraction process for the TMP product desired.
According to U.S. Pat. No. 5,948,943 to Supplee et al., crude TMP is heated in a mixture of an organic solvent and water forming a heated one phase solution, which solution is allowed to cool and to separate into at least two phases. TMP recovered from the aqueous solution exhibits an improved color. WO 2008/116826 discloses a process for the production of TMP with a low color number by treatment with activated carbon.
U.S. Pat. No. 6,117,277 to Zgorzelski et al. discloses a general process for purifying alcohols via distillation of said alcohols in the presence of small quantities of alkali metal hydroxides.
U.S. Pat. No. 7,126,018 to Poppe discloses a process for the production of polyol esters. The process consists of an esterification of a polyol, such as propylene glycol, and a fatty acid ester, such as a vegetable oil fatty acid methyl ester, in the presence of a catalyst and borohydride to yield a polyol ester having an improved color.
Further, U.S. Pat. No. 6,586,642 to Dernbach et al. teaches a hydrogenation of TMP, which has already been purified by distillation, for improving the color index.
Other prior art references relating to TMP production and color issues are U.S. Pat. Nos. 3,097,245; 3,185,274; 4,514,578; 6,034,285; 6,034,284; 6,187,971; 6,344,592; 6,692,616; 7,057,080; 7,211,701; and 7,253,326. All these references and any cited references cited herein are to be considered as incorporated herein by reference in toto.
Other processes have produced a product having various acid wash colors. However, for many applications, it is desirable to obtain a TMP product having lower acid wash colors, or other color analyses, as compared to the starting crude TMP and this low color obtained in an economically efficient manner. Thus, the art is continuously searching for methods to obtain low color TMP with increased efficiencies.